The present invention relates to an improved tubular frame for a mobile unit, and the use of improved connection members or elements for joining tubular elements of a frame. The present invention further relates to a method of deforming tubular frame elements to create improved joints in the frame and features that provide improved comfort during riding on the frame. More particularly, the present invention relates to the deforming of tubular frame elements, preferably through the use of electromagnetic pulse forming, to shape the elements into a structural mechanical joint. The joint and joint forming process permits connection of the tubular elements to each other without welding and preferably via a lug formed from a different material. The deformation process of the present invention, such as by electromagnetic pulse forming, may also be used to create ride compliance features that modify the stiffness of the tubular frame to increase rider comfort.
Mobile units, such as bicycles, wheelchairs, and walkers, are typically formed from tubular elements (e.g. having circular, elliptical, and tear drop cross-sections) connected to form a frame for the unit. The tubular elements may be formed of any desired material that provides the required strength to support the user or rider of the unit or device. Such materials include iron, steel, titanium, titanium alloys, aluminum, aluminum alloys, and various composite materials such as graphite epoxy plastic or other carbon fiber-based or composite materials.
Frames formed from aluminum tubing generally have welded connections between the individual tubes. However, welding anneals the material, and thus may weaken the material. For example, welding of aluminum alloy tubes often damages the structural strength of the frame. In order to strengthen the heat affected area at a welded joint, the thickness of the tubular element is generally increased in the area of the weld. The increase in material thickness adds to the weight of the frame. For these reasons, welding is not a desired method for connecting tubes formed of materials that are generally weaker than steel.
There is a growing trend toward use of lightweight materials for the tubular frame elements of mobile units such as bicycles. Often lightweight tubular elements are connected together to form a frame through the use of lugs. The materials of both the tubular elements and the lugs that are used to connect the tubular elements are preferably selected to be as lightweight, yet sturdy, as possible. Thus, it would be desirable to use different materials for the tubes and the lugs to thereby provide the strongest and lightest tubes and lugs possible. For example, composite or plastic lugs may be used to connect aluminum tubes. However, dissimilar materials cannot be welded. They must therefore be joined by bonding. However, conventional bonding does not always result in the strongest of joints between these different materials. Therefore, mobile units typically have tubular elements and lugs that are formed from the same, or similar materials.
Options for bonding tubular elements of a frame to lugs of different materials include screw joints, adhesives, and lug joints. With screw joints, the tubes are screwed into internally threaded lugs. Because one frame tube connector may receive more than one tube, it may be difficult to connect a plurality of frame tubes to a plurality of frame tube connectors by screw joints. Further, sufficient material thickness must exist to tap and thread adjoining tubes, resulting in increased weight. Also a threaded joint is weak in fatigue strength.
The use of adhesives to connect frame tubes is common. However, the process of applying and curing the bonding agent may be rather time consuming. Moreover, the quality of such joints is not easily inspectable, nor consistently good, and the adhesive or bonding agent that is used may weaken over time.
Lug joints are another method of bonding frame tubes and include forming an inside step on the inside surface of a hollow joint of a lug, and an outside, mating step on the outside surface of the frame tube. The frame tube is then inserted into the joint of the lug and the steps engage with each other. Because precision is crucial for this connection method, this method may be particularly expensive and not always reliable if not precisely achieved.
Yet another method of connecting frame tubes with lug joints involves the provision of through holes within a lug, and the deforming of the frame tube within the lug so it is outwardly projected into the through holes of the lug joint. This method, as disclosed in U.S. Pat. No. 5,404,630 to Wu, involves inserting an elastomer element into the frame tube and squeezing the elastomer element from its opposite ends to deform the wall of the frame tube into the holes of the lug joint. A reinforcing tube located within the frame tube may also be used with this method. The reinforcing tube is similarly deformed by the elastomer element and is caused to project through the holes of the lug with the frame tube. This process, accordingly, is time consuming and rather complicated.
The application of a high-energy magnetic pulse to deform or reshape a metal object is known in the art. Such plastic deformation of a metal object is known as electromagnetic forming and is described in, for example, U.S. Pat. No. 5,188,177 to Curry, and a number of other U.S. patents referenced therein. However, the Curry patent only discloses the use of such metal shaping to seal the open end of a tube, and not to form secure joints between tubes. The use of electromagnetic forming to joint metal tubes to metal joints is described in, for example, U.S. Pat. No. 3,837,759 to Benoit et al, and U.S. Pat. No. 4,523,872 to Arena et al. However, these patents relate to the joining of metal structural rods of aircrafts to each other and concentrate on forming strong torque joints. Thus, these patents do not teach joining the lightest and strongest tubes to the lightest and strongest lugs in frames of mobile units, such as bicycles, and do not focus on preventing relative axial movement between the joined elements.
It would accordingly be desirable to join the lightest yet sufficiently sturdy tubular elements (typically metal such as aluminum) with the lightest yet sufficiently sturdy lug possible (typically plastic, or a composite material) without encountering the above-described disadvantages.
As discussed above, in order to reduce the weight of the frame of mobile units such as bicycles, lightweight materials are substituted for conventional high strength, heavy materials such as steel. Aluminum tubing is commonly used to form lightweight bicycles. Because of the reduced yield strength of lightweight materials, such as aluminum, relative to high strength materials, such as steel, the diameters of the frame tubing of lightweight materials must be larger in order to achieve the required frame strength. As a result of the larger diameter tubing, lightweight frames have been criticized by their riders as being too stiff and providing too harsh a ride.